Composite materials formed by shear mixing of carbon nanostructures and related methods

ABSTRACT

Carbon nanostructures free of an adhered growth substrate can include a plurality of carbon nanotubes that are branched, crosslinked, and share common walls with one another. Under applied shear, crosslinks between the carbon nanotubes in carbon nanostructures can break to form fractured carbon nanotubes that are branched and share common walls. Methods for making polymer composites from carbon nanostructures can include combining a polymer matrix and a plurality of carbon nanostructures that are free of an adhered growth substrate, and dispersing the carbon nanostructures in the polymer matrix under applied shear. The applied shear breaks crosslinks between the carbon nanotubes to form a plurality of fractured carbon nanotubes that are dispersed as individuals in the polymer matrix. Polymer composites can include a polymer matrix and a plurality of fractured carbon nanotubes dispersed as individuals in the polymer matrix.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/040,529, filed on Sep. 27, 2013, which claims the benefit of priorityunder 35 U.S.C. §119 from U.S. Provisional Patent Applications61/707,738, filed on Sep. 28, 2012, and 61/722,085, filed on Nov. 2,2012, each of which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

The present disclosure generally relates to carbon nanostructures, and,more particularly, to polymer composites formed using carbonnanostructures.

BACKGROUND

Carbon nanotubes (CNTs) have been proposed for use in a number ofapplications that can take advantage of their unique combination ofchemical, mechanical, electrical, and thermal properties. In manyinstances, these properties can be tailored to the requirements of aparticular application by adjusting any combination of carbon nanotubelength, diameter, chirality, functionality, and like structuralfeatures. Various difficulties have been widely recognized in manyapplications when working with individual carbon nanotubes. Thesedifficulties can include, but are not limited to, poor solventsolubility, limited dispersibility in composite matrices, inadequatepurity, and the like. Without being bound by any theory or mechanism, itis believed that many of these issues can arise due to the strong vander Waals forces that occur between individual carbon nanotubes, therebycausing them to group into bundles or ropes, as known in the art. Theextreme aspect ratio of carbon nanotubes can also lead to physicalentanglement that can further contribute to these difficulties. Theforegoing issues and others can often result in lower than anticipatedproperty enhancements and/or inconsistent performance when individualcarbon nanotubes are employed in a chosen application. Although thereare various techniques available for de-bundling carbon nanotube ropes,bundles or agglomerates into individual, well-separated members, many ofthese techniques can detrimentally impact the desirable propertyenhancements that pristine carbon nanotubes are able to provide. Inaddition, widespread concerns have been raised regarding theenvironmental health and safety profile of individual carbon nanotubesdue to their small size. Furthermore, the cost of producing individualcarbon nanotubes may be prohibitive for the commercial viability ofthese entities in many instances.

In view of the foregoing, techniques for producing well-separated,individual carbon nanotubes while addressing at least some of the otherforegoing difficulties would be highly desirable in the art. The presentdisclosure satisfies the foregoing needs and provides related advantagesas well.

SUMMARY

In some embodiments, the present disclosure provides methods for makingpolymer composites. The methods include combining a plurality of carbonnanostructures and a polymer matrix, and dispersing the carbonnanostructures in the polymer matrix under applied shear. The carbonnanostructures are each free of an adhered growth substrate and containa plurality of carbon nanotubes that are branched, crosslinked, andshare common walls with one another. The applied shear breaks crosslinksbetween the carbon nanotubes to form a plurality of fractured carbonnanotubes that are dispersed as individuals in the polymer matrix. Theplurality of fractured carbon nanotubes contains carbon nanotubes thatare branched and share common walls with one another.

In some embodiments, the present disclosure provides polymer compositesformed from carbon nanostructures. In some embodiments, the polymercomposites can be formed by methods including combining a plurality ofcarbon nanostructures and a polymer matrix, and dispersing the carbonnanostructures in the polymer matrix under applied shear. The carbonnanostructures are each free of an adhered growth substrate and containa plurality of carbon nanotubes that are branched, crosslinked, andshare common walls with one another. The applied shear breaks crosslinksbetween the carbon nanotubes to form a plurality of fractured carbonnanotubes that are dispersed as individuals in the polymer matrix. Theplurality of fractured carbon nanotubes contains carbon nanotubes thatare branched and share common walls with one another.

In some embodiments, polymer composites can include a polymer matrix,and a plurality of fractured carbon nanotubes that are dispersed asindividuals in the polymer matrix. The plurality of fractured carbonnanotubes includes carbon nanotubes that are branched and share commonwalls with one another.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter, which form the subject of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIGS. 1A-1C show illustrative depictions of carbon nanotubes that arebranched, crosslinked, and share common walls, respectively;

FIG. 2 shows a comparative volume resistivity plot for a carbonnanostructure composite material and a multi-walled carbon nanotubecomposite material;

FIG. 3 shows a SEM image of an illustrative polymer composite containingintact carbon nanostructures encapsulated in a polymer matrix;

FIG. 4 shows an illustrative depiction of a carbon nanostructure flakematerial after isolation of the carbon nanostructure from a growthsubstrate;

FIG. 5 shows a SEM image of an illustrative carbon nanostructureobtained as a flake material;

FIG. 6 shows a flow diagram of an illustrative carbon nanostructuregrowth process which employs an exemplary glass or ceramic growthsubstrate;

FIG. 7 shows an illustrative schematic of a transition metalnanoparticle coated with an anti-adhesive layer;

FIG. 8 shows a flow diagram of an illustrative process for isolatingcarbon nanostructures from a growth substrate;

FIG. 9 shows an illustrative schematic further elaborating on theprocess demonstrated in FIG. 8;

FIG. 10 shows an illustrative schematic demonstrating how mechanicalshearing can be used to remove carbon nanostructures and a transitionmetal nanoparticle catalyst from a growth substrate; and

FIG. 11 shows an illustrative schematic demonstrating a carbonnanostructure removal process in which carbon nanostructures can beisolated from a growth substrate absent a transition metal nanoparticlecatalyst.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to composite materialsformed from carbon nanostructures, particularly polymer composites. Thepresent disclosure is also directed, in part, to methods for formingcomposite materials starting from carbon nanostructures, particularlypolymer composites.

As discussed above, various difficulties can sometimes be encountered inthe production and use of individual carbon nanotubes in manyapplications. Formation of composite materials containing carbonnanotubes, especially polymer composites, represents one instance whereat least some of the above difficulties can be particularly prevalent.Specifically, when forming composite materials, it can often bedifficult to effectively debundle carbon nanotube ropes in order toachieve an adequate carbon nanotube dispersion in a composite matrix.Moreover, surfactants used in conjunction with debundling carbonnanotube ropes can sometimes become incorporated in a composite matrixand degrade the resultant composite properties over those that couldotherwise have been attained.

In order to provide carbon nanotubes in an easily handled form thataddresses many of the foregoing issues, at least some of the presentinventors previously developed techniques to prepare carbonnanostructures infused to various fiber materials through direct growthof the carbon nanostructures thereon. As used herein, the term “carbonnanostructure” refers to a plurality of carbon nanotubes that can existas a polymeric structure by being interdigitated, branched, crosslinked,and/or sharing common walls with one another. Carbon nanostructures canbe considered to have carbon nanotubes as a base monomer unit of theirpolymeric structure. By growing carbon nanostructures on a substrate(e.g., a fiber material) under carbon nanostructure growth conditions,at least a portion of the carbon nanotubes in the carbon nanostructurescan be aligned substantially parallel to one another, much like theparallel carbon nanotube alignment seen in conventional carbon nanotubeforests. Carbon nanostructure-infused fiber materials can provide carbonnanotubes in a form that is much more easily handled than areunsupported carbon nanotubes. Infused carbon nanotubes in the form ofcarbon nanostructures also can represent a much lower environmentalhealth and safety concern than do free carbon nanotubes, which can formsub-micron particles with potential inhalation toxicity effects.Moreover, infusing carbon nanostructures to a fiber material can stillallow the beneficial carbon nanotube properties (i.e., any combinationof chemical, mechanical, electrical, and thermal properties) to beconveyed to the fiber material and/or a matrix material in which thecarbon nanostructure-infused fiber material is disposed.

Conventional carbon nanotube growth processes have most often focused onthe production of high purity carbon nanotubes containing a minimumnumber of defects. While such conventional carbon nanotube growthprocesses typically take several minutes or more to produce carbonnanotubes having micron-scale lengths, the carbon nanostructure growthprocesses described herein employ a nominal carbon nanotube growth rateon the order of several microns per second in a continuous, in situgrowth process on a growth substrate. As a result, the carbon nanotubeswithin the carbon nanostructures are more defective compared to aconventional carbon nanotube forest or unsupported carbon nanotubes.That is, the resultant carbon nanostructures contain carbon nanotubesthat are highly entangled, branched, crosslinked, and share commonwalls, thereby forming a macrostructure that is defined by more thanjust the structural features of carbon nanotubes themselves. Moreover,the ability to grow carbon nanostructures continuously on a growthsubstrate under such rapid growth conditions can provide access to muchgreater quantities of carbon nanostructures than can related carbonnanotube growth processes, thereby improving process economics. In mostcases, prior preparations of carbon nanostructure-infused fibermaterials have resulted in very robust adherence of the carbonnanostructures to the fiber material, such that the carbonnanostructures are not easily removed from the fiber material, at leastwithout significantly damaging the carbon nanotubes themselves.

Although carbon nanostructure-infused fiber materials can be usedsatisfactorily as a replacement for individual carbon nanotubes in manyapplications, at least some of the present inventors recognized that insome instances it might be more desirable to utilize carbonnanostructures that are free of the fiber material upon which they aregrown (i.e., “free” carbon nanostructures), while retaining the readycarbon nanotube handling attributes afforded by having the carbonnanostructures infused to the fiber material. Techniques for affectingremoval of carbon nanostructures from a growth substrate are set forthhereinbelow and are described in more detail in commonly owned U.S.patent application Ser. No. 14/035,856 entitled “Carbon Nanostructuresand Methods for Making the Same,” filed on Sep. 24, 2013 andincorporated herein by reference in its entirety.

In regard to the above, carbon nanostructures can be removed from theirgrowth substrate as a low density carbon nanostructure flake or likeparticulate material. The features of branching, crosslinking, andsharing common walls among the carbon nanotubes can be preserved whenthe carbon nanostructures are removed from a growth substrate, such thatthe carbon nanotubes are in a pre-exfoliated (i.e., at least partiallyseparated) state. The combination of these features can make the carbonnanostructures readily dispersible in a fluid medium without the need toinitially disperse the carbon nanotubes into individuals through use ofa surfactant. The pre-exfoliated state of the carbon nanotubes in thecarbon nanostructures can also make them more readily dispersible in acomposite matrix, as discussed in more detail hereinbelow.

Another advantage of carbon nanostructures over individual carbonnanotubes is that even “free” carbon nanostructures are believed toprovide a better environmental health and safety profile compared toindividual carbon nanotubes. Because carbon nanostructures aremacroscopic in size relative to an individual carbon nanotube, it isbelieved that even “free” carbon nanostructures can present fewertoxicity concerns and rival the environmental health and safety profileof infused carbon nanotubes. Without being bound by any theory, it isbelieved that the improved environmental health and safety profile canresult, at least in part, from the size and relative structuralintegrity of the carbon nanostructures themselves. That is, the bondinginteractions between carbon nanotubes in the carbon nanostructures canprovide a robust material that does not readily separate into harmfulsubmicron particulates, such as those associated with respirationtoxicity.

As a further advantage of carbon nanostructures relative to individualcarbon nanotubes, it is believed that carbon nanostructures can beproduced more rapidly and inexpensively and with a higher carbonfeedstock conversion percentage than can related carbon nanotubeproduction techniques. Some of the best performing carbon nanotubegrowth processes to date have exhibited a carbon conversion efficiencyof at most about 60%. In contrast, carbon nanostructures can be producedon a fiber material with carbon conversion efficiencies of greater thanabout 85%. Thus, carbon nanostructures provide a more efficient use ofcarbon feedstock material and associated lower production costs.

Moreover, due to their different and sometimes superior propertiescompared to carbon nanotubes, lower amounts of “free” carbonnanostructures can be used in some applications to achieve a comparableeffect only seen with higher quantities of individual carbon nanotubes.Without being bound by any theory or mechanism, it is believed thatimproved dispersion of the carbon nanostructures and the resultantproperty expression in various matrices can afford the superiorperformance of carbon nanostructures over carbon nanotubes. Remainingunbound by any theory or mechanism, it is believed that the enhanceddispersion of carbon nanostructures results from their low densityrelative to individual carbon nanotubes.

In much of the description that follows hereinbelow, reference is madeto polymer composites. However, it is to be recognized that inalternative embodiments, other types of composite materials such as forexample, ceramic matrix composites, can be formed in a like manner.

One application in which “free” carbon nanostructures can provideparticular benefits is that of forming polymer composites or other typesof composite materials. Although carbon nanostructure-infused fibermaterials can be advantageously employed in this regard, the fibermaterial often simply represents a carrier to facilitate handling anddeployment of the carbon nanostructures in the polymer matrix. However,the fiber material can also provide mechanical reinforcement, even whencoated with infused carbon nanostructures. When the reinforcementcontribution of the fiber material is not needed in a polymer composite(e.g., the conveyed properties of the carbon nanotubes are instead ofprimary interest), the fiber material can represent an unwanted weightcontribution to the polymer composite. Particularly in aeronautic andaerospace applications, the weight contribution from the fiber materialcan be significant, especially if its reinforcement effects are notneeded in a given location of a polymer composite.

Moreover, the fabrication of certain types of parts made fromfiber-containing polymer composites can be quite problematic. Forexample, extruded filaments and injection-molded parts can be difficultto fabricate from polymer matrices containing a fiber material. Incontrast, these difficulties are significantly reduced when the fibermaterial is absent and only nanomaterials are present.

In some embodiments, an unmodified fiber material (i.e., a commercialfiber material, with or without a sizing coated thereon, that is notinfused with carbon nanostructures, carbon nanotubes, or othernanomaterials) can be incorporated in the composites described herein.For example, when a localized enhancement of properties is desired in apolymer composite, a fiber material (either an unmodified fiber materialor a carbon nanostructure-infused fiber material, for example) can beincluded only in regions of the polymer composite where mechanicalstrength is needed, and carbon nanostructures or a product formedtherefrom can be included only in regions of the polymer composite wherenanomaterial enhancement is needed, thereby allowing tailoring of thecomposite's properties. Thus, the enhancement effects provided by thefiber material and the carbon nanostructures can be effectivelyseparated in the polymer composite. Moreover, because “free” carbonnanostructures and a fiber material can be introduced separately into apolymer matrix, differing ratios of these components can be utilizedthan if only carbon nanostructure-infused fiber materials were used,thereby allowing further tailoring of the composite's properties to takeplace.

Given the above benefits, the present inventors investigated thepreparation of polymer composites from carbon nanostructures separatedfrom the growth substrate upon which they were formed (e.g., a fibermaterial). In some cases, enhanced composite properties could berealized in polymer composites containing carbon nanostructures relativeto comparable polymer composites containing an equal weight of carbonnanotubes (for example, see FIG. 2). In order to obtain well-dispersedcarbon nanostructures, the inventors employed high shear mixing of thecarbon nanostructures and the polymer matrix in some embodiments.Surprisingly, the inventors found that high shear mixing resulted inalteration of the initial carbon nanostructure morphology. Specifically,the inventors found that high shear mixing resulted in the breaking ofcrosslinks between the carbon nanotubes within the carbon nanostructuresto produce carbon nanotubes dispersed as individuals in the polymermatrix. The structural features of carbon nanotube branching and sharedwalls were predominantly retained, even after the crosslinks wereremoved under high shear mixing. Thus, the predominant structuralfeatures of the fractured carbon nanotubes formed from carbonnanostructures differ from those in conventional carbon nanotube polymercomposites prepared by other techniques. As used herein, the term“fractured carbon nanotubes” will refer to carbon nanotubes preparedfrom carbon nanostructures under shear, in which the structural featuresof carbon nanotube branching and shared walls are predominantlyretained. Such fractured carbon nanotubes can readily be differentiatedfrom ordinary carbon nanotubes through standard carbon nanotubeanalytical techniques, such as SEM, for example.

In addition to the composite structure produced under shear mixing ofcarbon nanostructures, the inventors recognized that utilizing carbonnanostructures to produce individual carbon nanotubes in a polymercomposite also presented further advantages over conventional compositeformation techniques from carbon nanotube ropes or bundles. Foremost,the pre-exfoliated state of the carbon nanostructures can allowindividual carbon nanotubes to be produced without utilizing asurfactant to debundle the carbon nanotubes from each other, therebypotentially improving the composite's properties. Further, sinceindividual carbon nanotubes are produced in situ within the polymermatrix and remain trapped therein, potential inhalation toxicity effectscan be reduced. Moreover, due to their higher solubility than carbonnanotube ropes, carbon nanostructures can allow access to higherquantities of individualized carbon nanotubes in a polymer matrix thancan nanocomposites produced by other techniques, thereby potentiallyimproving strength, stiffness, thermal conductivity and/or electricalconductivity as the quantity of the carbon nanotubes increases. Finally,the techniques described herein are compatible with high end materialssuch as, for example, ultra high molecular weight polyethylene andaramid polymers, thereby simplifying the formation of polymer compositesfrom these materials. However, it is to be recognized that the foregoingadvantages can also be realized in more traditional composite matricesas well, in the form of decreased production costs and shortermanufacturing times.

In various embodiments, polymer composites described herein can beformed from a plurality of carbon nanostructures. The carbonnanostructures can each include a plurality of carbon nanotubes in whichthe carbon nanotubes are branched, crosslinked, and share common wallswith one another. It is to be recognized that every carbon nanotube inthe plurality of carbon nanotubes does not necessarily have theforegoing structural features of branching, crosslinking, and sharingcommon walls. Rather, the plurality of carbon nanotubes as a whole canpossess one or more of these structural features. That is, in someembodiments, at least a portion of the carbon nanotubes are branched, atleast a portion of the carbon nanotubes are crosslinked, and at least aportion of the carbon nanotubes share common walls. FIGS. 1A-1C showillustrative depictions of carbon nanotubes 1-3 that are branched,crosslinked, and share common walls, respectively. The carbon nanotubesin the carbon nanostructures can be formed with branching, crosslinking,and sharing common walls with one another during formation of the carbonnanostructures on a growth substrate. Moreover, during formation of thecarbon nanostructures on a growth substrate, the carbon nanotubes can beformed such that they are substantially parallel to one another in thecarbon nanostructures. The carbon nanostructures can be considered to bea polymer having a carbon nanotube as a base monomer unit that is inparallel alignment with at least some other carbon nanotubes. It is tobe further understood that every carbon nanotube in the carbonnanostructures need not necessarily be branched, crosslinked, or sharecommon walls with other carbon nanotubes. For example, in someembodiments, at least a portion of the carbon nanotubes in the carbonnanostructures can be interdigitated with one another and/or withbranched, crosslinked, or common wall carbon nanotubes in the remainderof the carbon nanostructure.

In some embodiments, the carbon nanostructures can have a web-likemorphology that results in the carbon nanostructures having a lowinitial bulk density. As-produced carbon nanostructures can have aninitial bulk density ranging between about 0.003 g/cm³ to about 0.015g/cm³. Further consolidation and coating to produce a carbonnanostructure flake or like material can raise the initial bulk densityto a range between about 0.1 g/cm³ to about 0.15 g/cm³. In someembodiments, optional further modification of the carbon nanostructurescan be conducted to further alter the bulk density and/or anotherproperty of the carbon nanostructures. In some embodiments, the initialbulk density of the carbon nanostructures can be further altered byforming a coating on the carbon nanotubes of the carbon nanostructuresand/or infiltrating the interior of the carbon nanostructures withvarious materials. Coating the carbon nanotubes and/or infiltrating theinterior of the carbon nanostructures can also be used to further tailorthe properties of the carbon nanostructures for use in variousapplications. Moreover, in some embodiments, forming a coating on thecarbon nanotubes can desirably facilitate the handling of the carbonnanostructures. Further compaction can raise the bulk density to anupper limit of about 1 g/cm³, with chemical modifications to the carbonnanostructure raising the bulk density to an upper limit of about 1.2g/cm³.

The polymer composites described herein can contain intact carbonnanostructures, or at least a portion of the carbon nanostructures canbreak down under applied shear to form fractured carbon nanotubes, asgenerally described above. Although polymer composites containingfractured carbon nanotubes are believed to present particularadvantages, including those noted above, polymer composites containingintact carbon nanostructures are also contemplated by variousembodiments of the present disclosure. In general, such polymercomposites can include a polymer matrix and a plurality of carbonnanostructures dispersed in a polymer matrix. For example, if the carbonnanostructures are dispersed in a polymer matrix without applyingsufficient shear to significantly break apart the carbon nanostructures,a carbon nanostructure polymer composite can be obtained. As describedabove, such polymer composites can display superior properties over likepolymer composites containing a comparable weight carbon nanotubes. Forexample, FIG. 2 shows a comparative volume resistivity plot for a carbonnanostructure composite material and a multi-walled carbon nanotubecomposite material. As shown in FIG. 2, comparable volume resistivitiesin the composite material can be obtained with as low as ¼^(th) to⅕^(th) the weight percentage of carbon nanostructures relative tomulti-walled carbon nanotubes. It is not believed that the differingmatrix materials in the tested composite samples have an appreciableimpact on their measured volume resistivities. In various embodiments,the type of matrix material employed in the polymer composites describedherein is not believed to be particularly limited and can include, forexample, thermoplastic, thermosetting, or elastomeric matrices,illustrative examples of each of which are provided below. FIG. 3 showsa SEM image of an illustrative polymer composite containing intactcarbon nanostructures encapsulated in a polymer matrix.

In some embodiments, the carbon nanostructures can be in the form of aflake material after being removed from the growth substrate upon whichthe carbon nanostructures are initially formed. As used herein, the term“flake material” refers to a discrete particle having finite dimensions.FIG. 4 shows an illustrative depiction of a carbon nanostructure flakematerial after isolation of the carbon nanostructure from a growthsubstrate. Flake structure 100 can have first dimension 110 that is in arange from about 1 nm to about 35 μm thick, particularly about 1 nm toabout 500 nm thick, including any value in between and any fractionthereof. Flake structure 100 can have second dimension 120 that is in arange from about 1 micron to about 750 microns tall, including any valuein between and any fraction thereof. Flake structure 100 can have thirddimension 130 that is only limited in size based on the length of thegrowth substrate upon which the carbon nanostructures are initiallyformed. For example, in some embodiments, the process for growing carbonnanostructures on a growth substrate can take place on a tow or rovingof a fiber-based material of spoolable dimensions. The carbonnanostructure growth process can be continuous, and the carbonnanostructures can extend the entire length of a spool of fiber. Thus,in some embodiments, third dimension 130 can be in a range from about 1m to about 10,000 m wide. Again, third dimension 130 can be very longbecause it represents the dimension that runs along the axis of thegrowth substrate upon which the carbon nanostructures are formed. Thirddimension 130 can also be decreased to any desired length less than 1 m.For example, in some embodiments, third dimension 130 can be on theorder of about 1 micron to about 10 microns, or about 10 microns toabout 100 microns, or about 100 microns to about 500 microns, or about500 microns to about 1 cm, or about 1 cm to about 100 cm, or about 100cm to about 500 cm, up to any desired length, including any amountbetween the recited ranges and any fractions thereof. Since the growthsubstrate upon which the carbon nanostructures are formed can be quitelarge, exceptionally high molecular weight carbon nanostructures can beproduced by forming the polymer-like morphology of the carbonnanostructures as a continuous layer on a suitable growth substrate.

Referring still to FIG. 4, flake structure 100 can include a webbednetwork of carbon nanotubes 140 in the form of a carbon nanotube polymer(i.e., a “carbon nanopolymer”) having a molecular weight in a range fromabout 15,000 g/mol to about 150,000 g/mol, including all values inbetween and any fraction thereof. In some embodiments, the upper end ofthe molecular weight range can be even higher, including about 200,000g/mol, about 500,000 g/mol, or about 1,000,000 g/mol. The highermolecular weights can be associated with carbon nanostructures that aredimensionally long. In various embodiments, the molecular weight canalso be a function of the predominant carbon nanotube diameter andnumber of carbon nanotube walls present within the carbonnanostructures. In some embodiments, the carbon nanostructures can havea crosslinking density ranging between about 2 mol/cm³ to about 80mol/cm³. The crosslinking density can be a function of the carbonnanostructure growth density on the surface of the growth substrate aswell as the carbon nanostructure growth conditions.

FIG. 5 shows a SEM image of an illustrative carbon nanostructureobtained as a flake material. The carbon nanostructure shown in FIG. 5exists as a three dimensional microstructure due to the entanglement andcrosslinking of its highly aligned carbon nanotubes. The alignedmorphology is reflective of the formation of the carbon nanotubes on agrowth substrate under rapid carbon nanotube growth conditions (e.g.,several microns per second, such as about 2 microns per second to about10 microns per second), thereby inducing substantially perpendicularcarbon nanotube growth from the growth substrate. Without being bound byany theory or mechanism, it is believed that the rapid rate of carbonnanotube growth on the growth substrate can contribute, at least inpart, to the complex structural morphology of the carbon nanostructure.In addition, the bulk density of the carbon nanostructure can bemodulated to some degree by adjusting the carbon nanostructure growthconditions, including, for example, by changing the concentration oftransition metal nanoparticle catalyst particles that are disposed onthe growth substrate to initiate carbon nanotube growth. Suitabletransition metal nanoparticle catalysts and carbon nanostructure growthconditions are outlined in more detail below.

As discussed above, it is believed that polymer composites formed fromcarbon nanostructures under applied shear conditions can presentparticular advantages for dispersing individualized carbon nanotubes inthe polymer matrix. In various embodiments, such polymer composites caninclude a polymer matrix, and a plurality of fractured carbon nanotubesthat are dispersed as individuals in the polymer matrix, in which theplurality of fractured carbon nanotubes includes carbon nanotubes thatare branched and share common walls with one another. Again, it is to berecognized that every carbon nanotube in the plurality of fracturedcarbon nanotubes need not necessarily be branched and share commonwalls, rather that the plurality of fractured carbon nanotubes as awhole possesses these features. The carbon nanostructures used to formthe polymer composites can be in any carbon nanostructure form describedherein, such as a carbon nanostructure flake material, for example.

The form of the polymer composites containing the fractured carbonnanotubes is not believed to be particularly limited. In someembodiments, the polymer composites can be in the form of a polymercomposite sheet. In some embodiments, the polymer composites can be inthe form of a composite coating on an article. In some embodiments, thepolymer composites can be in the form of a composite filament or aplurality of filaments, particularly extruded filaments. In someembodiments, composite filaments can be produced by extruding a melt ofthe polymer matrix and the fractured carbon nanostructures followingapplication of a shearing force thereto. In some embodiments, aplurality of filaments can be produced in a two-dimensional orthree-dimensional structure, such as a prepreg fabric structure. In someembodiments, the polymer composites can be in the form of a solidarticle, such as those produced by molding, extrusion, or pultrusiontechnologies.

In some embodiments, a polymer matrix, such as a softened thermoplasticpolymer, can be mixed with carbon nanostructures under applied shear toproduce fractured carbon nanotubes dispersed throughout thethermoplastic polymer. Thereafter, in some embodiments, thethermoplastic polymer can be chopped into pellets, and the pellets canthen be molded to form an article. In some embodiments, molding thepellets to form an article can take place by injection molding or pressmolding. In some embodiments, the thermoplastic polymer pellets can bemixed with polymer pellets lacking fractured carbon nanostructuresbefore undergoing molding. Thus, the quantity of fractured carbonnanostructures in the composites can be readily regulated. Shear mixingof carbon nanostructures with an elastomer or a thermosetting resin cantake place in a similar manner. In each embodiment after dispersal offractured carbon nanotubes in the polymer matrix, the polymer matrix canbe hardened, such as after forming a desired article.

In various embodiments, at least a portion of the fractured carbonnanotubes can be functionalized. Suitable reactions for functionalizingcarbon nanotubes will be familiar to one having ordinary skill in theart. The fractured carbon nanotubes can be functionalized after theirformation from carbon nanostructures under applied shear, or the carbonnanostructures can be functionalized before being placed under appliedshear, with the introduced functionality being carried forward into thefractured carbon nanotubes. Suitable processes for functionalizing thecarbon nanotubes of carbon nanostructures are usually similar to thosethat can be used to functionalize individual carbon nanotubes and willbe familiar to a person having ordinary skill in the art. In variousembodiments, suitable techniques for functionalizing the carbonnanotubes of carbon nanostructures can include, for example, reactingthe carbon nanostructures with an oxidant, such as KMnO₄, H₂O₂, HNO₃ orany combination thereof.

In some embodiments, at least a portion of the fractured carbonnanotubes can be covalently bonded to the polymer matrix. Such covalentbonding can take place through functional groups installed on thefractured carbon nanotubes, as described above, or a component of thepolymer matrix can react directly with the fractured carbon nanotubes toform a covalent bond thereto. Covalent bonding between the fracturedcarbon nanotubes and the polymer matrix can result in a more effectiveconveyance of their beneficial properties to the polymer composite.

In some embodiments, the polymer composites described herein can alsoinclude a fiber material incorporated in the polymer matrix, in whichthe fiber material is not adhered to the fractured carbon nanotubes. Insome embodiments, the fiber material can be a fiber material that is notmodified with carbon nanostructures or carbon nanotubes. That is, insome embodiments, a commercial glass, carbon, ceramic, metal, or organicfiber (e.g., an aramid fiber) can be included in the polymer composites,if desired. Such commercial fibers can be sized or unsized. For example,in some embodiments, it can be desirable to introduce a commercial sizedfiber into a polymer composite containing fractured carbon nanotubes, ifmechanical reinforcement or other property enhancements are needed froma particular type of fiber. Likewise, in some embodiments, a fibermaterial can be incorporated in portions of a polymer composite in whichfractured carbon nanotubes are not present, or different concentrationsa fiber material can be incorporated in portions of a polymer compositewhere fractured carbon nanotubes are present versus where they are not.In some embodiments, the fiber material can be a carbon nanotube-infusedfiber material.

In some embodiments, a fiber material can be introduced into the polymercomposites unintentionally, particularly as a residual effect ofremoving carbon nanostructures from a growth substrate. As describedhereinbelow, during removal of carbon nanostructures from a fibermaterial, fragments of the fiber material can become liberated from thebulk fiber and admix with the “free” carbon nanostructures. Althoughvarious methods for separating carbon nanostructures from fragments of anon-adhered fiber material are described hereinbelow, residual fragmentsof the fiber material can also become incorporated in the polymercomposites described herein, particularly if operations are notundertaken to separate the fiber material fragments from the carbonnanostructures.

In general, it is not believed that the concentration of fracturedcarbon nanotubes in the polymer composites described herein isparticularly limited, other than being constrained by the quantity ofcarbon nanostructures that can be effectively sheared in a givenquantity of polymer matrix. In some embodiments, effective shearing cantake place when the polymer composite contains up to about 60% carbonnanostructures by weight. Accordingly, in some embodiments, the polymercomposites can contain up to about 60% fractured carbon nanotubes byweight. In more particular embodiments, the polymer composites cancontain between about 0.1% to about 15% fractured carbon nanotubes byweight. Within the foregoing range, various properties such as enhancedelectrical conductivity and mechanical strength can be conveyed to thepolymer composites via the fractured carbon nanotubes. In still moreparticular embodiments, a concentration of fractured carbon nanotubes inthe polymer composites can range between about 3% to about 6% of thepolymer composite by weight, or between about 1% to about 5% of thepolymer composite by weight, or between about 2% to about 6% of thepolymer composite by weight, or between about 5% to about 10% of thepolymer composite by weight, or between about 10% to about 15% of thepolymer composite by weight.

In some embodiments, polymer composites formed from carbonnanostructures can incorporate different amounts of carbonnanostructures or fractured carbon nanotubes in certain portions of thepolymer matrix. That is, in some embodiments, a concentration of carbonnanostructures or fractured carbon nanotubes can vary throughout thepolymer composite in a gradient manner. A gradient concentration canvary throughout the polymer composite in a continuous gradient, suchthat there is a steady increase or decrease in concentration throughoutthe composite in a particular direction, or the gradient can be stepped,such that the concentration increases or decreases in a non-continuousmanner. In some embodiments, certain portions of the polymer compositescan lack fractured carbon nanostructures altogether. In otherembodiments, a concentration of carbon nanostructures or fracturedcarbon nanotubes throughout the polymer composite can be uniform.

Various additives can also be found in or on the carbon nanostructuresfrom which the polymer composites are formed. Additives that can bepresent include, but are not limited to, a coating on the carbonnanotubes, a filler material in the interstitial space of the carbonnanostructures, transition metal nanoparticles, and any combinationthereof. In some embodiments, certain additives can be covalently bondedto at least a portion of the carbon nanotubes in at least some of thecarbon nanostructures.

In some embodiments, the polymer composites described herein can includea coating on the fractured carbon nanotubes. The coating can differ incomposition from the polymer matrix of the polymer composite. Coatingscan be applied to the carbon nanotubes of the carbon nanostructuresbefore or after removal of the carbon nanostructures from a growthsubstrate. Application of a coating before removal of the carbonnanostructures from a growth substrate can, for example, protect thecarbon nanotubes during the removal process or facilitate the removalprocess. In other embodiments, a coating can be applied to the carbonnanotubes of the carbon nanostructures after removal of the carbonnanostructures from their growth substrates. Regardless of when thecoating is applied to the carbon nanotubes of the carbon nanostructures,the coating can be carried forward and remain on the fractured carbonnanotubes following the application of shear. In some embodiments, thecoating on the fractured carbon nanotubes can include, for example, ametal coating or a ceramic coating. In some embodiments, the coating canbe covalently bonded to the fractured carbon nanotubes via the carbonnanotube functionalization techniques discussed above.

In some or other embodiments, there can be a filler or other additivematerial present in at least the interstitial space between the carbonnanotubes of the carbon nanostructures (i.e., on the interior of thecarbon nanostructures). The additive material can be present alone or incombination with a coating on the carbon nanotubes of the carbonnanostructures. When used in combination with a coating, the additivematerial can also be located on the exterior of the carbonnanostructures within the coating, in addition to being located withinthe interstitial space of the carbon nanostructures. Introduction of anadditive material within the interstitial space of the carbonnanostructures or elsewhere within the carbon nanostructures can resultin further modification of the properties of the carbon nanostructures.Without limitation, the inclusion of an additive material within thecarbon nanostructures can result in modification of the carbonnanostructure's density, thermal properties, spectroscopic properties,mechanical strength, and the like. It is not believed that individual orbundled carbon nanotubes are capable of carrying an additive material ina like manner, since they lack a permanent interstitial space on thenanotube exterior to contain the additive material. Although there isempty space on the carbon nanotube interior, it is believed to be eithervery difficult or impossible to place an additive material in thatlocation.

In various embodiments, an additive associated with the carbonnanostructures can become dispersed in a polymer matrix under appliedshear. That is, as the carbon nanostructures undergo shear, the additivecan be released into the polymer matrix and undergo dispersal. However,in some embodiments, certain additives can remain associated with thefractured carbon nanotubes without being released. For example, somecoatings on the carbon nanotubes may not be released and dispersed intothe polymer matrix, while other types of coatings may be released.Similarly, covalently bound additives may not be readily released intothe polymer matrix upon shearing.

In some embodiments, the carbon nanostructures can contain a pluralityof transition metal nanoparticles, where the transition metalnanoparticles can represent a catalyst that was used in synthesizing thecarbon nanostructures. In some embodiments, the transition metalnanoparticles can be coated with an anti-adhesive coating that limitstheir adherence to a growth substrate or the carbon nanostructure to agrowth substrate, as shown in FIG. 7. Suitable anti-adhesive coatingsare discussed in more detail below. In various embodiments, theanti-adhesive coating can be carried along with the transition metalnanoparticles as the carbon nanostructures and the transition metalnanoparticles are removed from a growth substrate. In other embodiments,the anti-adhesive coating can be removed from the transition metalnanoparticles before or after they are incorporated into the carbonnanostructures. In still other embodiments, the transition metalnanoparticles can initially be incorporated into the carbonnanostructures and then subsequently removed. For example, in someembodiments, at least a portion of the transition metal nanoparticlescan be removed from the carbon nanostructures by treating the carbonnanostructures with a mineral acid.

In various embodiments, the polymer matrix of the polymer compositesdescribed herein can be a thermoplastic polymer. Suitable thermoplasticpolymers can include, for example, polypropylenes, polyethylenes,polyamides, polyesters, polycarbonates, polysulfones, aramid polymers,polyamide-imides, polyimides, polyetherimides, polyetheretherketones,and polyphenylene sulfides. In some embodiments, the polymer compositesdisclosed herein can employ a thermoplastic polymer such as, forexample, polyethylene, polypropylene, polyethylene terephthalate,polybutylene terephthalate, polystyrene, polyvinyl chloride, polyvinylalcohol, polyvinyl acetate, polyacrylonitrile, polyethyleneimine, andcopolymers thereof. In some such embodiments, the thermoplastic polymercan be a low-end thermoplastic such as, for example,acrylonitrile-butadiene-styrene copolymer (ABS), a polycarbonate, anylon, or any combination thereof. Techniques for mixing thethermoplastic polymer and the carbon nanostructures can include, forexample, melt impregnation, solvent impregnation, powder impregnation,and the like.

In various embodiments, the polymer matrix of the polymer compositesdescribed herein can be a thermosetting polymer, also known as athermosetting resin polymer or just a thermosetting resin. Suitablethermosetting resins can include, for example, phthalic/maleic typepolyesters, vinyl ester polymers, epoxies, phenolics, cyanates,silicones, urethanes, bismaleimides, and nadic end-capped polyimides(e.g., PMR-15). The carbon nanostructures can be dispersed in athermosetting resin using any suitable technique, and the thermosettingresin can then be cured to a hardened polymer thereafter. Suitablepolymer composite processing techniques for thermosetting resins caninclude, for example, fiber layup, resin transfer molding, wet winding,vacuum assisted resin transfer molding, and prepreg manufacture.

In various embodiments, suitable elastomers for use in the polymercomposites described herein can include, for example: (1) unsaturatedrubbers; (a) natural polyisoprene: (i) cis-1,4-polyisoprene naturalrubber, and (ii) trans-1,4-polyisoprene; (b) synthetic polyisoprene; (c)polybutadiene; (d) chloroprene rubber, such as polychloroprene(including NEOPRENE, BAYPREN, and the like); (e) butyl rubber (copolymerof isobutylene and isoprene); (f) halogenated butyl rubbers; (g)styrene-butadiene rubber (copolymer of styrene and butadiene); (h)nitrile rubber (copolymer of butadiene and acrylonitrile); and (i)hydrogenated nitrile rubbers (including THERBAN and ZETPOL); and (2)saturated rubbers; (a) ethylene propylene rubber, a copolymer ofethylene and propylene (EPM) and ethylene propylene diene rubber, aterpolymer of ethylene, propylene and a diene-component (EPDM); (b)epichlorohydrin rubber (ECO); (c) polyacrylic rubber; (d) siliconerubber; (e) fluorosilicone rubber; (f) fluoroelastomers (includingVITON, TECNOFLON, FLUOREL, and DAI-EL); (g) perfluoroelastomers(including TECNOFLON PFR, KALREZ, CHEMRAZ, and PERLAST); (h) polyetherblock amides (PEBA); (i) chlorosulfonated polyethylene, such as HYPALON;and (j) ethylene-vinyl acetate (EVA).

In various embodiments, polymer composites described herein can beprepared by methods that include combining a plurality of carbonnanostructures and a polymer matrix, and dispersing the carbonnanostructures in the polymer matrix under applied shear. The appliedshear breaks crosslinks between the carbon nanotubes to form a pluralityof fractured carbon nanotubes that are dispersed as individuals in thepolymer matrix. The plurality of fractured carbon nanotubes containcarbon nanotubes that are branched and share common walls with oneanother.

In some embodiments, the applied shear can completely break down thecarbon nanostructures, such that the resultant polymer compositescontain only fractured carbon nanotubes. In other embodiments, breakdown of the carbon nanostructures can be incomplete, such that theresultant polymer composites contain a mixture of fractured carbonnanotubes and intact carbon nanostructures.

In some embodiments, the methods described herein can further includehardening the polymer matrix after dispersing the fractured carbonnanotubes as individuals therein. For a thermoplastic matrix, hardeningcan involve cooling the thermoplastic matrix below its glass transitiontemperature. For a thermosetting resin, hardening can involve curing thethermosetting resin to form a polymer.

In some embodiments, methods described herein can further includeincorporating a fiber material in the polymer matrix. The type of fibermaterial is not believed to be particularly limited and can include, forexample, glass, ceramic, carbon, metal or aramid fibers. In someembodiments, the fiber material can be a sized or unsized commercialfiber material. In other embodiments, the fiber material can be a carbonnanostructure-infused fiber material. Suitable techniques forincorporating a fiber material into the polymer matrix can include, forexample, blending, laying up, and the like.

Although the foregoing embodiments have been described in reference topolymer composites, it is to be recognized that other types of compositematerials are also contemplated by the present disclosure. In someembodiments, as an alternative to polymer matrices, the compositematerials described herein can contain a ceramic matrix, a carbonmatrix, or a metal matrix.

In some embodiments, methods described herein can further includeforming carbon nanostructures on a growth substrate, and removing thecarbon nanostructures from the growth substrate prior to combining thecarbon nanostructures with the polymer matrix. Production of carbonnanostructures on a growth substrate and subsequent removal therefrom byvarious techniques are now further described hereinbelow.

In some embodiments, processes described herein can include preparingcarbon nanostructures on a growth substrate with one or more provisionsfor removal of the carbon nanostructures once synthesis of the carbonnanostructures is complete. The provision(s) for removing the carbonnanostructures from the growth substrate can include one or moretechniques selected from the group consisting of: (i) providing ananti-adhesive coating on the growth substrate, (ii) providing ananti-adhesive coating on a transition metal nanoparticle catalystemployed in synthesizing the carbon nanostructures, (iii) providing atransition metal nanoparticle catalyst with a counter ion that etchesthe growth substrate, thereby weakening the adherence of the carbonnanostructures to the growth substrate, and (iv) conducting an etchingoperation after carbon nanostructure synthesis is complete to weakenadherence of the carbon nanostructures to the growth substrate.Combinations of these techniques can also be used. In combination withthese techniques, various fluid shearing or mechanical shearingoperations can be carried out to affect the removal of the carbonnanostructure from the growth substrate.

In some embodiments, processes disclosed herein can include removingcarbon nanostructures from a growth substrate. In some embodiments,removing carbon nanostructures from a growth substrate can include usinga high pressure liquid or gas to separate the carbon nanostructures fromthe growth substrate, separating contaminants derived from the growthsubstrate (e.g., fragmented growth substrate) from the carbonnanostructures, collecting the carbon nanostructures with air or from aliquid medium with the aid of a filter medium, and isolating the carbonnanostructures from the filter medium. In various embodiments,separating contaminants derived from the growth substrate from thecarbon nanostructures can take place by a technique selected from thegroup consisting of cyclone filtering, density separation, size-basedseparation, and any combination thereof. The foregoing processes aredescribed in more detail hereinbelow.

FIG. 6 shows a flow diagram of an illustrative carbon nanostructuregrowth process 400, which employs an exemplary glass or ceramic growthsubstrate 410. It is to be understood that the choice of a glass orceramic growth substrate is merely exemplary, and the substrate can alsobe metal, an organic polymer (e.g., aramid), basalt fiber, or carbon,for example. In some embodiments, the growth substrate can be a fibermaterial of spoolable dimensions, thereby allowing formation of thecarbon nanostructures to take place continuously on the growth substrateas the growth substrate is conveyed from a first location to a secondlocation. Carbon nanostructure growth process 400 can employ growthsubstrates in a variety of forms such as fibers, tows, yarns, woven andnon-woven fabrics, sheets, tapes, belts and the like. For convenience incontinuous syntheses, tows and yarns are particularly convenient fibermaterials.

Referring still to FIG. 6, such a fiber material can be meted out from apayout creel at operation 420 and delivered to an optional desizingstation at operation 430. Desizing is ordinarily conducted whenpreparing carbon nanostructure-infused fiber materials in order toincrease the degree of infusion of the carbon nanostructures to thefiber material. However, when preparing isolated carbon nanostructures,desizing operation 430 can be skipped, for example, if the sizingpromotes a decreased degree of adhesion of the transition metalnanoparticle catalyst and/or carbon nanostructures to the growthsubstrate, thereby facilitating removal of the carbon nanostructure.Numerous sizing compositions associated with fiber substrates cancontain binders and coupling agents that primarily provide anti-abrasiveeffects, but typically do not exhibit exceptional adhesion to fibersurface. Thus, forming carbon nanostructures on a growth substrate inthe presence of a sizing can actually promote subsequent isolation ofthe carbon nanostructures in some embodiments. For this reason, it canbe beneficial to skip desizing operation 430, in some embodiments.

In some embodiments, an additional coating application can take place atoperation 440. Additional coatings that can be applied in operation 440include, for example, colloidal ceramics, glass, silanes, or siloxanesthat can decrease catalyst and/or carbon nanostructure adhesion to thegrowth substrate. In some embodiments, the combination of a sizing andthe additional coating can provide an anti-adhesive coating that canpromote removal of the carbon nanostructures from the growth substrate.In some embodiments, the sizing alone can provide sufficientanti-adhesive properties to facilitate carbon nanostructure removal fromthe growth substrate, as discussed above. In some embodiments, theadditional coating provided in operation 440 alone can providesufficient anti-adhesive properties to facilitate carbon nanostructureremoval from the growth substrate. In still further embodiments, neitherthe sizing nor the additional coating, either alone or in combination,provides sufficient anti-adhesive properties to facilitate carbonnanostructure removal. In such embodiments, decreased adhesion of thecarbon nanostructures to the growth substrate can be attained byjudicious choice of the transition metal nanoparticles used to promotegrowth of the carbon nanostructures on the growth substrate.Specifically, in some such embodiments, operation 450 can employ acatalyst that is specifically chosen for its poor adhesivecharacteristics.

Referring still to FIG. 6, after optional desizing operation 430 andoptional coating operation 440, catalyst is applied to the growthsubstrate in operation 450, and carbon nanostructure growth is affectedthrough a small cavity CVD process in operation 460. The resultingcarbon nanostructure-infused growth substrate (i.e., a carbonnanostructure-infused fiber material) can be wound for storage andsubsequent carbon nanostructure removal or immediately taken into acarbon nanostructure isolation process employing a harvester, asindicated in operation 470.

In some embodiments, the growth substrate can be modified to promoteremoval of carbon nanostructures therefrom. In some embodiments, thegrowth substrate used for producing carbon nanostructures can bemodified to include an anti-adhesive coating that limits adherence ofthe carbon nanostructures to the growth substrate. The anti-adhesivecoating can include a sizing that is commercially applied to the growthsubstrate, or the anti-adhesive coating can be applied after receipt ofthe growth substrate. In some embodiments, a sizing can be removed fromthe growth substrate prior to applying an anti-adhesive coating. Inother embodiments, a sizing can be applied to a growth substrate inwhich a sizing is present.

In some embodiments, the carbon nanostructures can be grown on thegrowth substrate from a catalyst that includes a plurality of transitionmetal nanoparticles, as generally described hereinbelow. In someembodiments, one mode for catalyst application onto the growth substratecan be through particle adsorption, such as through direct catalystapplication using a liquid or colloidal precursor-based deposition.Suitable transition metal nanoparticle catalysts can include any d-blocktransition metal or d-block transition metal salt. In some embodiments,a transition metal salt can be applied to the growth substrate withoutthermal treatments. In other embodiments, a transition metal salt can beconverted into a zero-valent transition metal on the growth substratethrough a thermal treatment.

In some embodiments, the transition metal nanoparticles can be coatedwith an anti-adhesive coating that limits their adherence to the growthsubstrate. As discussed above, coating the transition metalnanoparticles with an anti-adhesive coating can also promote removal ofthe carbon nanostructures from the growth substrate following synthesisof the carbon nanostructures. Anti-adhesive coatings suitable for use inconjunction with coating the transition metal nanoparticles can includethe same anti-adhesive coatings used for coating the growth substrate.FIG. 7 shows an illustrative schematic of a transition metalnanoparticle coated with an anti-adhesive layer. As shown in FIG. 7,coated catalyst 500 can include core catalyst particle 510 overcoatedwith anti-adhesive layer 520. In some embodiments, colloidalnanoparticle solutions can be used in which an exterior layer about thenanoparticle promotes growth substrate to nanoparticle adhesion butdiscourages carbon nanostructure to nanoparticle adhesion, therebylimiting adherence of the carbon nanostructures to the growth substrate.

FIG. 8 shows a flow diagram of an illustrative process for isolatingcarbon nanostructures from a growth substrate. As shown in FIG. 8,process 600 begins with a carbon nanostructure-infused fiber beingprovided in operation 610. Non-fibrous growth substrates onto which acarbon nanostructure has been grown can be used in a like manner. Fluidshearing can be conducted at operation 620 using a gas or a liquid inorder to accomplish removal of the carbon nanostructures from the fibermaterial. In some cases, fluid shearing can result in at least a portionof the fiber material being liberated from the bulk fiber andincorporated with the free carbon nanostructure, while not being adheredthereto. If needed, in operation 630, the liberated carbon nanostructurecan be subjected to cyclonic/media filtration in order to remove thenon-adhered fiber material fragments. Density-based or size-basedseparation techniques can also be used to bring about separation of thecarbon nanostructures from the non-adhered fiber material. In the caseof gas shearing, the carbon nanostructure can be collected in dry formon a filter medium in operation 645. The resultant dry flake materialcollected in operation 645 can be subjected to any optional furtherchemical or thermal purification, as outlined further in FIG. 8. In thecase of liquid shearing, the liquid can be collected in operation 640,and separation of the carbon nanostructure from the liquid can takeplace in operation 650, ultimately producing a dry flake material inoperation 660. The carbon nanostructure flake material isolated inoperation 660 can be similar to that produced in operation 645. Afterisolating the carbon nanostructure flake material in operation 660, itcan be ready for packaging and/or storage in operation 695. In processesemploying gas shearing to remove the carbon nanostructure, the carbonnanostructures can be dry collected in a filter at operation 645. Priorto packaging and/or storage in operation 695, the crude product formedby either shearing technique can undergo optional chemical and/orthermal purification in operation 670. These purification processes canbe similar to those conducted when purifying traditional carbonnanotubes. By way of example, purification conducted in operation 670can involve removal of a catalyst used to affect carbon nanostructuregrowth, such as, for example, through treatment with liquid bromine.Other purification techniques can be envisioned by one having ordinaryskill in the art.

Referring still to FIG. 8, the carbon nanostructures produced by eithershearing technique can undergo further processing by cutting or fluffingin operation 680. Such cutting and fluffing can involve mechanical ballmilling, grinding, blending, chemical processes, or any combinationthereof. Further optionally, in operation 690, the carbon nanostructurescan be further functionalized using any technique in which carbonnanotubes are normally modified or functionalized. Suitablefunctionalization techniques in operation 690 can include, for example,plasma processing, chemical etching, and the like. Functionalization ofthe carbon nanostructures in this manner can produce chemical functionalgroup handles that can be used for further modifications. For example,in some embodiments, a chemical etch can be employed to form carboxylicacid groups on the carbon nanostructures that can be used to bring aboutcovalent attachment to any number of further entities including, forexample, the matrix material of a composite material. In this regard, afunctionalized carbon nanostructure can provide a superior reinforcementmaterial in a composite matrix, since it can provide multiple sites forcovalent attachment to the composite's matrix material in alldimensions, or provide a source for fractured carbon nanotubes that canalso do the same.

In addition to facilitating the covalent attachment of carbonnanostructures to the matrix of a composite material, functionalizationof carbon nanostructures can also allow other groups to be covalentlyattached to the carbon nanostructures. In some embodiments, access toother covalently linked entities such as synthetic or biopolymers can berealized via functional group handles produced in post-processing carbonnanostructure functionalization. For example, carbon nanostructures canbe linked to polyethylene glycol (e.g., through ester bonds formed fromcarboxylic acid groups on the carbon nanostructures) to providePEGylated carbon nanostructures, which can confer improved watersolubility to the carbon nanostructures. In some embodiments, the carbonnanostructures can provide a platform for covalent attachment tobiomolecules to facilitate biosensor manufacture. In this regard, thecarbon nanostructures can provide improved electrical percolationpathways for enhanced detection sensitivity relative to other carbonnanotube-based biosensors employing individualized carbon nanotubes oreven conventional carbon nanotube forests. Biomolecules of interest forsensor development can include, for example, peptides, proteins,enzymes, carbohydrates, glycoproteins, DNA, RNA, and the like.

FIG. 9 shows an illustrative schematic further elaborating on theprocess demonstrated in FIG. 8. As illustrated in process 700 of FIG. 9,a single spool or multiple spools of a carbon nanostructure-ladenfiber-type substrate is fed in operation 710 to removal chamber 712using a pay-out and take-up system. Removal of the carbon nanostructuresfrom the fiber-type substrate can be affected with a single or severalpressurized air source tools 714, such as an air knife or air nozzle atoperation 720. Such air source tools can be placed generallyperpendicular to the spool(s), and the air can then be directed on tothe fiber-type substrate carrying the carbon nanostructures. In someembodiments, the air source tool can be stationary, while in otherembodiments, the air source tool can be movable. In embodiments wherethe air source tool is movable, it can be configured to oscillate withrespect to the surface of the fiber-type substrate to improve theremoval efficiency. Upon air impact, fiber tows and other bundledfiber-type substrates can be spread, thereby exposing additional surfacearea on the substrate and improving removal of the carbonnanostructures, while advantageously avoiding mechanical contact. Insome embodiments, the integrity of the substrate can be sufficient torecycle the substrate in a continuous cycle of carbon nanostructuresynthesis and removal. Thus, in some embodiments, the substrate can bein the form of a belt or a loop in which carbon nanostructures aresynthesized on the substrate, subsequently removed downstream, and thenrecycled for additional growth of new carbon nanostructures in thelocation where the original carbon nanostructures were removed. In someembodiments, removal of the original carbon nanostructures can result inremoval of the surface treatment that facilitated carbon nanostructureremoval. Thus, in some embodiments, the substrate can again be modifiedafter removal of the original carbon nanostructures to promote removalof the new carbon nanostructures, as generally performed according tothe surface modification techniques described herein. The surfacetreatment performed on the substrate after the original carbonnanostructures are removed can be the same or different as the originalsurface treatment.

In some embodiments, the integrity of the substrate can be compromisedduring carbon nanostructure removal, and at least a portion of thesubstrate can become admixed with the carbon nanostructures while nolonger being adhered thereto. Referring still to FIG. 9, fragmentedsubstrate that has become admixed with the isolated carbonnanostructures can be removed in operation 730. In FIG. 9, operation 730is depicted as taking place by cyclonic filtration, but any suitablesolids separation technique can be used. For example, in someembodiments, sieving, differential settling, or other size-basedseparations can be performed. In other embodiments, density-basedseparations can be performed. In still other embodiments, a chemicalreaction may be used, at least in part, to affect separation of thecarbon nanostructures from growth substrate that is not adhered to thecarbon nanostructures. Although FIG. 9 has depicted a single cyclonicfiltration, multiple vacuum and cyclonic filtration techniques can beused in series, parallel, or any combination thereof to remove residualfragmented growth substrate from the carbon nanostructures. Suchtechniques can employ multiple stages of filter media and/or filtrationrates to selectively capture the fragmented growth substrate whileallowing the carbon nanostructures to pass to a collection vessel. Theresultant carbon nanostructures can be either collected dry at operation740 or collected as a wet sludge at operation 750. In some embodiments,the carbon nanostructures can be processed directly following theremoval of fragmented growth substrate in operation 730 and packed intoa storage vessel or shippable container in packaging operation 760.Otherwise, packaging can follow dry collection operation 740 or wetcollection operation 750.

In embodiments where wet processing is employed, the carbonnanostructures can be mixed with and captured in a solution containingabout 1% to about 40% solvent in water and passed through a filter orlike separation mechanism to separate the carbon nanostructure from thewater/solvent solution. The resultant separated carbon nanostructurescan be dried and packed or stored “wet” as a dispersion in a fluidphase. It has been observed that unlike individualized carbon nanotubesolutions or dispersions, carbon nanostructures can advantageously formstable dispersions. In some embodiments, stable dispersions can beachieved in the absence of stabilizing surfactants, even with water assolvent. In some or other embodiments, a solvent can be used incombination with water during wet processing. Suitable solvents for usein conjunction with wet processing can include, but are not limited to,isopropanol (IPA), ethanol, methanol, and water.

In various embodiments, the CNS described herein can be removed from thegrowth substrate in intact form by a shearing action imparted on the CNSby directed and focused fluid media or mechanical contact. As analternative to fluid shearing, mechanical shearing can be used to removethe carbon nanostructure from the growth substrate in some embodiments.FIG. 10 shows an illustrative schematic demonstrating how mechanicalshearing can be used to remove a carbon nanostructure and a transitionmetal nanoparticle catalyst from a growth substrate. As shown in FIG.10, carbon nanostructure removal process 800 can employ mechanicalshearing force 810 to remove both the carbon nanostructure and thetransition metal nanoparticle catalyst from growth substrate 830 asmonolithic entity 820. In some such embodiments, sizing and/oradditional anti-adhesive coatings can be employed to limit carbonnanostructure and/or nanoparticle adhesion to the growth substrate,thereby allowing mechanical shear or another type of shearing force tofacilitate removal of the carbon nanostructure from the growthsubstrate. In some embodiments, mechanical shear can be provided bygrinding the carbon nanostructure-infused fiber with dry ice.

As another alternative to fluid shearing, in some embodiments,sonication can be used to remove the carbon nanostructures from thegrowth substrate.

In some embodiments, the carbon nanostructures can be removed from thegrowth substrate without substantially removing the transition metalnanoparticle catalyst. FIG. 11 shows an illustrative schematicdemonstrating carbon nanostructure removal process 900 in which a carbonnanostructure can be isolated from a growth substrate absent atransition metal nanoparticle catalyst. As shown in FIG. 11, carbonnanostructure 940 can be grown on growth substrate 920 using implantedtransition metal nanoparticle catalyst 910. Thereafter, shear removal930 of carbon nanostructure 940 leaves transition metal nanoparticlecatalyst 910 behind on growth substrate 920. In some such embodiments, alayered catalyst can promote adhesion to the substrate surface, whiledecreasing carbon nanostructure to nanoparticle adhesion.

Although FIGS. 10 and 11 have depicted carbon nanostructure growth astaking place with basal growth from the catalyst, the skilled artisanwill recognize that other mechanistic forms of carbon nanostructuregrowth are possible. For example, carbon nanostructure growth can alsotake place such that the catalyst resides distal to the growth substrateon the surface of the carbon nanostructure (i.e., tip growth) orsomewhere between tip growth and basal growth. In some embodiments,predominantly basal growth can be selected to aid in carbonnanostructure removal from the growth substrate.

In alternative embodiments, removal of the carbon nanostructures fromthe growth substrate can take place by a process other than fluidshearing or mechanical shearing. In some embodiments, chemical etchingcan be used to remove the carbon nanostructures from the growthsubstrate. In some embodiments, the transition metal nanoparticlecatalyst used to promote carbon nanostructure growth can be a transitionmetal salt containing an anion that is selected to etch the growthsubstrate, thereby facilitating removal of the carbon nanostructures.Suitable etching anions can include, for example, chlorides, sulfates,nitrates, nitrites, and fluorides. In some or other embodiments, achemical etch can be employed independently from the catalyst choice.For example, when employing a glass substrate, a hydrogen fluoride etchcan be used to weaken adherence of the carbon nanostructures and/or thetransition metal nanoparticle catalyst to the substrate.

The carbon nanostructures disclosed herein comprise carbon nanotubes(CNTs) in a network having a complex structural morphology, which hasbeen described in more detail hereinabove. Without being bound by anytheory or mechanism, it is believed that this complex structuralmorphology results from the preparation of the carbon nanostructures ona substrate under CNT growth conditions that produce a rapid growth rateon the order of several microns per second. The rapid CNT growth rate,coupled with the close proximity of the CNTs to one another, can conferthe observed branching, crosslinking, and shared wall motifs to theCNTs. In the discussion that follows, techniques for producing carbonnanostructures bound to a fiber substrate are described. For simplicity,the discussion may refer to carbon nanostructures disposed on thesubstrate interchangeably as CNTs, since CNTs represent the majorstructural component of carbon nanostructures.

In some embodiments, the processes disclosed herein can be applied tonascent fiber materials generated de novo before, or in lieu of,application of a typical sizing solution to the fiber material.Alternatively, the processes disclosed herein can utilize a commercialfiber material, for example, a tow, that already has a sizing applied toits surface. In such embodiments, the sizing can be removed to provide adirect interface between the fiber material and the synthesized carbonnanostructure, although a transition metal nanoparticle catalyst canserve as an intermediate linker between the two. After carbonnanostructure synthesis, further sizing agents can be applied to thefiber material as desired. For the purpose of carbon nanostructureisolation, any of the above mentioned sizing or coatings can be employedto facilitate the isolation process. Equally suitable substrates forforming a carbon nanostructure include tapes, sheets and even threedimensional forms which can be used to provide a shaped carbonnanostructure product. The processes described herein allow for thecontinuous production of CNTs that make up the carbon nanostructurenetwork having uniform length and distribution along spoolable lengthsof tow, tapes, fabrics and other 3D woven structures.

As used herein the term “fiber material” refers to any material whichhas fiber as its elementary structural component. The term encompassesfibers, filaments, yarns, tows, tows, tapes, woven and non-wovenfabrics, plies, mats, and the like.

As used herein the term “spoolable dimensions” refers to fiber materialshaving at least one dimension that is not limited in length, allowingfor the material to be stored on a spool or mandrel. Processes ofdescribed herein can operate readily with 5 to 20 lb. spools, althoughlarger spools are usable. Moreover, a pre-process operation can beincorporated that divides very large spoolable lengths, for example 100lb. or more, into easy to handle dimensions, such as two 50 lb. spools.

As used herein, the term “carbon nanotube” (CNT, plural CNTs) refers toany of a number of cylindrically-shaped allotropes of carbon of thefullerene family including single-walled carbon nanotubes (SWNTs),double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes(MWNTs). CNTs can be capped by a fullerene-like structure or open-ended.CNTs include those that encapsulate other materials. CNTs can appear inbranched networks, entangled networks, and combinations thereof. TheCNTs prepared on the substrate within the carbon nanostructure caninclude individual CNT motifs from exclusive MWNTs, SWNTs, or DWNTs, orthe carbon nanostructure can include mixtures of CNT these motifs.

As used herein “uniform in length” refers to an average length of CNTsgrown in a reactor for producing a carbon nanostructure. “Uniformlength” means that the CNTs have lengths with tolerances of plus orminus about 20% of the total CNT length or less, for CNT lengths varyingfrom between about 1 micron to about 500 microns. At very short lengths,such as 1-4 microns, this error may be in a range from between aboutplus or minus 20% of the total CNT length up to about plus or minus 1micron, that is, somewhat more than about 20% of the total CNT length.In the context of the carbon nanostructure, at least one dimension ofthe carbon nanostructure can be controlled by the length of the CNTsgrown.

As used herein “uniform in distribution” refers to the consistency ofdensity of CNTs on a growth substrate, such as a fiber material.“Uniform distribution” means that the CNTs have a density on the fibermaterial with tolerances of plus or minus about 10% coverage defined asthe percentage of the surface area of the fiber covered by CNTs. This isequivalent to ±1500 CNTs/μm² for an 8 nm diameter CNT with 5 walls. Sucha figure assumes the space inside the CNTs as Tillable.

As used herein, the term “transition metal” refers to any element oralloy of elements in the d-block of the periodic table. The term“transition metal” also includes salt forms of the base transition metalelement such as oxides, carbides, nitrides, and the like.

As used herein, the term “nanoparticle” or NP (plural NPs), orgrammatical equivalents thereof refers to particles sized between about0.1 to about 100 nanometers in equivalent spherical diameter, althoughthe NPs need not be spherical in shape. Transition metal NPs, inparticular, can serve as catalysts for CNT growth on the fibermaterials.

As used herein, the term “sizing agent,” “fiber sizing agent,” or just“sizing,” refers collectively to materials used in the manufacture offibers as a coating to protect the integrity of fibers, provide enhancedinterfacial interactions between a fiber and a matrix material in acomposite, and/or alter and/or enhance particular physical properties ofa fiber.

As used herein, the term “material residence time” refers to the amountof time a discrete point along a fiber material of spoolable dimensionsis exposed to CNT growth conditions during the CNS processes describedherein. This definition includes the residence time when employingmultiple CNT growth chambers.

As used herein, the term “linespeed” refers to the speed at which afiber material of spoolable dimensions is fed through the CNT synthesisprocesses described herein, where linespeed is a velocity determined bydividing CNT chamber(s)′ length by the material residence time.

In some embodiments, the CNT-laden fiber material includes a fibermaterial of spoolable dimensions and carbon nanotubes (CNTs) in the formof a carbon nanostructure grown on the fiber material.

Without being bound by any theory or mechanism, transition metal NPs,which serve as a CNT-forming catalyst, can catalyze CNT growth byforming a CNT growth seed structure. In one embodiment, the CNT-formingcatalyst can remain at the base of the fiber material (i.e., basalgrowth). In such a case, the seed structure initially formed by thetransition metal nanoparticle catalyst is sufficient for continuednon-catalyzed seeded CNT growth without allowing the catalyst to movealong the leading edge of CNT growth (i.e., tip growth). In such a case,the NP serves as a point of attachment for the CNS to the fibermaterial.

Compositions having CNS-laden fiber materials are provided in which theCNTs are substantially uniform in length. In the continuous processdescribed herein, the residence time of the fiber material in a CNTgrowth chamber can be modulated to control CNT growth and ultimately,CNT and CNS length. These features provide a means to control specificproperties of the CNTs grown and hence the properties of the CNS. CNTlength can also be controlled through modulation of the carbon feedstockand carrier gas flow rates and reaction temperature. Additional controlof the CNT properties can be obtained by modulating, for example, thesize of the catalyst used to prepare the CNTs. For example, 1 nmtransition metal nanoparticle catalysts can be used to provide SWNTs inparticular. Larger catalysts can be used to prepare predominantly MWNTs.

Additionally, the CNT growth processes employed are useful for providinga CNS-laden fiber material with uniformly distributed CNTs whileavoiding bundling and/or aggregation of the CNTs that can occur inprocesses in which pre-formed CNTs are suspended or dispersed in asolvent medium and applied by hand to the fiber material. In someembodiments, the maximum distribution density, expressed as percentcoverage, that is, the surface area of fiber material that is covered,can be as high as about 55% assuming about 8 nm diameter CNTs with 5walls. This coverage is calculated by considering the space inside theCNTs as being “fillable” space. Various distribution/density values canbe achieved by varying catalyst dispersion on the surface as well ascontrolling gas composition and process speed. Typically for a given setof parameters, a percent coverage within about 10% can be achievedacross a fiber surface. Higher density and shorter CNTs (e.g., less thanabout 100 microns in length) can be useful for improving mechanicalproperties, while longer CNTs (e.g., greater than about 100 microns inlength) with lower density can be useful for improving thermal andelectrical properties, although increased density still can befavorable. A lower density can result when longer CNTs are grown. Thiscan be the result of the higher temperatures and more rapid growthcausing lower catalyst particle yields.

CNS-laden fiber materials can include a fiber material such asfilaments, a fiber yarn, a fiber tow, a fiber-braid, a woven fabric, anon-woven fiber mat, a fiber ply, and other 3D woven structures.Filaments include high aspect ratio fibers having diameters ranging insize from between about 1 micron to about 100 microns. Fiber tows aregenerally compactly associated bundles of filaments and are usuallytwisted together to give yarns.

Yarns include closely associated bundles of twisted filaments. Eachfilament diameter in a yarn is relatively uniform. Yarns have varyingweights described by their ‘tex,’ expressed as weight in grams of 1000linear meters, or denier, expressed as weight in pounds of 10,000 yards,with a typical tex range usually being between about 200 tex to about2000 tex.

Tows include loosely associated bundles of untwisted filaments. As inyarns, filament diameter in a tow is generally uniform. Tows also havevarying weights and the tex range is usually between 200 tex and 2000tex. They are frequently characterized by the number of thousands offilaments in the tow, for example 12K tow, 24K tow, 48K tow, and thelike.

Tapes are materials that can be assembled as weaves or can representnon-woven flattened tows. Tapes can vary in width and are generallytwo-sided structures similar to ribbon. CNT infusion can take place onone or both sides of a tape. CNS-laden tapes can resemble a “carpet” or“forest” on a flat substrate surface. However, the CNS can be readilydistinguished from conventional aligned CNT forests due to thesignificantly higher degree of branching and crosslinking that occurs inthe CNS structural morphology. Again, processes described herein can beperformed in a continuous mode to functionalize spools of tape.

Fiber braids represent rope-like structures of densely packed fibers.Such structures can be assembled from yarns, for example. Braidedstructures can include a hollow portion or a braided structure can beassembled about another core material.

CNTs lend their characteristic properties such as mechanical strength,low to moderate electrical resistivity, high thermal conductivity, andthe like to the CNS-laden fiber material. For example, in someembodiments, the electrical resistivity of a carbon nanotube-laden fibermaterial is lower than the electrical resistivity of a parent fibermaterial. Likewise, such properties can translate to the isolated CNS.More generally, the extent to which the resulting CNS-laden fiberexpresses these characteristics can be a function of the extent anddensity of coverage of the fiber by the carbon nanotubes. Any amount ofthe fiber surface area, from 0-55% of the fiber can be covered assumingan 8 nm diameter, 5-walled MWNT (again this calculation counts the spaceinside the CNTs as fillable). This number is lower for smaller diameterCNTs and more for greater diameter CNTs. 55% surface area coverage isequivalent to about 15,000 CNTs/micron². Further CNT properties can beimparted to the fiber material in a manner dependent on CNT length, asdescribed above. CNTs within the carbon nanostructure can vary in lengthfrom between about 1 micron to about 500 microns, including about 1micron, about 2 microns, about 3 microns, about 4 micron, about 5,microns, about 6, microns, about 7 microns, about 8 microns, about 9microns, about 10 microns, about 15 microns, about 20 microns, about 25microns, about 30 microns, about 35 microns, about 40 microns, about 45microns, about 50 microns, about 60 microns, about 70 microns, about 80microns, about 90 microns, about 100 microns, about 150 microns, about200 microns, about 250 microns, about 300 microns, about 350 microns,about 400 microns, about 450 microns, about 500 microns, and all valuesand sub-ranges in between. CNTs can also be less than about 1 micron inlength, including about 0.5 microns, for example. CNTs can also begreater than 500 microns, including for example, about 510 microns,about 520 microns, about 550 microns, about 600 microns, about 700microns and all values and subranges in between. It will be understoodthat such lengths accommodate the presence of crosslinking and branchingand therefore the length may be the composite length measured from thebase of the growth substrate up to the edges of the CNS.

CNSs described herein can also incorporate CNTs have a length from about1 micron to about 10 microns. Such CNT lengths can be useful inapplication to increase shear strength. CNTs can also have a length fromabout 5 to about 70 microns. Such CNT lengths can be useful inapplications for increased tensile strength if the CNTs are aligned inthe fiber direction. CNTs can also have a length from about 10 micronsto about 100 microns. Such CNT lengths can be useful to increaseelectrical/thermal properties as well as mechanical properties. CNTshaving a length from about 100 microns to about 500 microns can also bebeneficial to increase electrical and thermal properties. Such controlof CNT length is readily achieved through modulation of carbon feedstockand inert gas flow rates coupled with varying linespeeds and growthtemperatures.

In some embodiments, compositions that include spoolable lengths ofCNS-laden fiber materials can have various uniform regions withdifferent lengths of CNTs. For example, it can be desirable to have afirst portion of CNS-laden fiber material with uniformly shorter CNTlengths to enhance shear strength properties, and a second portion ofthe same spoolable material with a uniform longer CNT length to enhanceelectrical or thermal properties.

Processes for rapid CNS growth on fiber materials allow for control ofthe CNT lengths with uniformity in continuous processes with spoolablefiber materials. With material residence times between 5 to 300 seconds,linespeeds in a continuous process for a system that is 3 feet long canbe in a range anywhere from about 0.5 ft/min to about 36 ft/min andgreater. The speed selected depends on various parameters as explainedfurther below.

In some embodiments, a material residence time of about 5 seconds toabout 30 seconds can produce CNTs having a length between about 1 micronto about 10 microns. In some embodiments, a material residence time ofabout 30 seconds to about 180 seconds can produce CNTs having a lengthbetween about 10 microns to about 100 microns. In still furtherembodiments, a material residence time of about 180 seconds to about 300seconds can produce CNTs having a length between about 100 microns toabout 500 microns. One skilled in the art will recognize that theseranges are approximate and that CNT length can also be modulated byreaction temperatures, and carrier and carbon feedstock concentrationsand flow rates.

In some embodiments, continuous processes for CNS growth can include (a)disposing a carbon nanotube-forming catalyst on a surface of a fibermaterial of spoolable dimensions; and (b) synthesizing carbon nanotubesdirectly on the fiber material, thereby forming a CNS-laden fibermaterial. For a 9 foot long system, the linespeed of the process canrange from between about 1.5 ft/min to about 108 ft/min. The linespeedsachieved by the process described herein allow the formation ofcommercially relevant quantities of CNS-laden fiber materials with shortproduction times. For example, at 36 ft/min linespeed, the quantities ofCNS-laden fibers (over 5% CNTs on fiber by weight) can exceed over 100pound or more of material produced per day in a system that is designedto simultaneously process 5 separate tows (20 lb/tow). Systems can bemade to produce more tows at once or at faster speeds by repeatinggrowth zones.

As described further below the catalyst can be prepared as a liquidsolution that contains CNT-forming catalyst that contains transitionmetal nanoparticles. The diameters of the synthesized nanotubes arerelated to the size of the transition metal nanoparticles as describedabove. In some embodiments, commercial dispersions of CNT-formingtransition metal nanoparticle catalysts are available and can be usedwithout dilution, and in other embodiments commercial dispersions ofcatalyst can be diluted. Whether to dilute such solutions can depend onthe desired density and length of CNT to be grown as described above.

Carbon nanotube synthesis can be based on a chemical vapor deposition(CVD) process and occurs at elevated temperatures. The specifictemperature is a function of catalyst choice, but will typically be in arange of about 500° C. to about 1000° C. This operation involves heatingthe fiber material to a temperature in the aforementioned range tosupport carbon nanotube synthesis.

CVD-promoted nanotube growth on the catalyst-laden fiber material isthen performed. The CVD process can be promoted by, for example, acarbon-containing feedstock gas such as acetylene, ethylene, methane,and/or propane. The CNT synthesis processes generally use an inert gas(nitrogen, argon, helium) as a primary carrier gas. The carbon feedstockis generally provided in a range from between about 0% to about 50% ofthe total mixture. A substantially inert environment for CVD growth isprepared by removal of moisture and oxygen from the growth chamber.

The operation of disposing a catalyst on the fiber material can beaccomplished by spraying or dip coating a solution or by gas phasedeposition via, for example, a plasma process. Thus, in someembodiments, after forming a solution of a catalyst in a solvent,catalyst can be applied by spraying or dip coating the fiber materialwith the solution, or combinations of spraying and dip coating. Eithertechnique, used alone or in combination, can be employed once, twice,thrice, four times, up to any number of times to provide a fibermaterial that is sufficiently uniformly coated with CNT-formingcatalyst. When dip coating is employed, for example, a fiber materialcan be placed in a first dip bath for a first residence time in thefirst dip bath. When employing a second dip bath, the fiber material canbe placed in the second dip bath for a second residence time. Forexample, fiber materials can be subjected to a solution of CNT-formingcatalyst for between about 3 seconds to about 90 seconds depending onthe dip configuration and linespeed. Employing spraying or dip coatingprocesses, a fiber material with a surface density of catalyst of lessthan about 5% surface coverage to as high as about 80% coverage, inwhich the CNT-forming catalyst nanoparticles are nearly monolayer. Insome embodiments, the process of coating the CNT-forming catalyst on thefiber material should produce no more than a monolayer. For example, CNTgrowth on a stack of CNT-forming catalyst can erode the degree ofinfusion of the CNT to the fiber material. In other embodiments, thetransition metal catalyst can be deposited on the fiber material usingevaporation techniques, electrolytic deposition techniques, and otherdeposition processes, such as addition of the transition metal catalystto a plasma feedstock gas as a metal organic, metal salt or othercomposition promoting gas phase transport.

Because processes for growing carbon nanostructures are designed to becontinuous, a spoolable fiber material can be dip-coated in a series ofbaths where dip coating baths are spatially separated. In continuousprocesses in which nascent fibers are being generated de novo, dip bathor spraying of CNT-forming catalyst can be the first step. In otherembodiments, the CNT-forming catalyst can be applied to newly formedfibers in the presence of other sizing agents. Such simultaneousapplication of CNT-forming catalyst and other sizing agents can providethe CNT-forming catalyst in the surface of the sizing on the fibermaterial to create a poorly adhered CNT coating.

The catalyst solution employed can be a transition metal nanoparticlewhich can be any d-block transition metal, as described above. Inaddition, the nanoparticles can include alloys and non-alloy mixtures ofd-block metals in elemental form or in salt form, and mixtures thereof.Such salt forms include, without limitation, oxides, carbides, acetates,and nitrides. Non-limiting exemplary transition metal NPs include Ni,Fe, Co, Mo, Cu, Pt, Au, and Ag and salts thereof and mixtures thereof.In some embodiments, such CNT-forming catalysts are disposed on thefiber by applying or infusing a CNT-forming catalyst directly to thefiber material simultaneously with barrier coating deposition. Many ofthese transition metal catalysts are readily commercially available froma variety of suppliers, including, for example, Sigma Aldrich (St.Louis, Mo.) or Ferrotec Corporation (Bedford, N.H.).

Catalyst solutions used for applying the CNT-forming catalyst to thefiber material can be in any common solvent that allows the CNT-formingcatalyst to be uniformly dispersed throughout. Such solvents caninclude, without limitation, water, acetone, hexane, isopropyl alcohol,toluene, ethanol, methanol, tetrahydrofuran (THF), cyclohexane or anyother solvent with controlled polarity to create an appropriatedispersion of the CNT-forming catalyst nanoparticles. Concentrations ofCNT-forming catalyst can be in a range from about 1:1 to 1:10000catalyst to solvent. Such concentrations can be used when the barriercoating and CNT-forming catalyst are applied simultaneously as well.

In some embodiments heating of the fiber material can be at atemperature that is between about 500° C. and about 1000° C. tosynthesize carbon nanotubes after deposition of the CNT-formingcatalyst. Heating at these temperatures can be performed prior to orsubstantially simultaneously with introduction of a carbon feedstock forCNT growth.

In some embodiments, the processes for producing a carbon nanostructureinclude removing a sizing agent from a fiber material, applying anadhesion-inhibiting coating (i.e., an anti-adhesive coating) conformallyover the fiber material, applying a CNT-forming catalyst to the fibermaterial, heating the fiber material to at least 500° C., andsynthesizing carbon nanotubes on the fiber material. In someembodiments, operations of the CNS-growth process can include removingsizing from a fiber material, applying an adhesion-inhibiting coating tothe fiber material, applying a CNT-forming catalyst to the fiber,heating the fiber to CNT-synthesis temperature and performingCVD-promoted CNS growth on the catalyst-laden fiber material. Thus,where commercial fiber materials are employed, processes forconstructing CNS-laden fibers can include a discrete step of removingsizing from the fiber material before disposing adhesion-inhibitingcoating and the catalyst on the fiber material.

Synthesizing carbon nanotubes on the fiber material can include numeroustechniques for forming carbon nanotubes, including those disclosed inco-pending U.S. Patent Application Publication No. 2004/0245088, whichis incorporated herein by reference. The CNS grown on the fibers can beformed by techniques such as, for example, micro-cavity, thermal orplasma-enhanced CVD techniques, laser ablation, arc discharge, and highpressure carbon monoxide (HiPCO). In some embodiments, any conventionalsizing agents can be removed prior CNT synthesis. In some embodiments,acetylene gas can be ionized to create a jet of cold carbon plasma forCNT synthesis. The plasma is directed toward the catalyst-bearing fibermaterial. Thus, in some embodiments for synthesizing CNS on a fibermaterial include (a) forming a carbon plasma; and (b) directing thecarbon plasma onto the catalyst disposed on the fiber material. Thediameters of the CNTs that are grown are dictated by the size of theCNT-forming catalyst as described above. In some embodiments, the sizedfiber material is heated to between about 550° C. to about 800° C. tofacilitate CNS synthesis. To initiate the growth of CNTs, two gases arebled into the reactor: a process gas such as argon, helium, or nitrogen,and a carbon-containing gas, such as acetylene, ethylene, ethanol ormethane. CNTs grow at the sites of the CNT-forming catalyst.

In some embodiments, the CVD growth is plasma-enhanced. A plasma can begenerated by providing an electric field during the growth process. CNTsgrown under these conditions can follow the direction of the electricfield. Thus, by adjusting the geometry of the reactor, verticallyaligned carbon nanotubes can be grown radially about a cylindricalfiber. In some embodiments, a plasma is not required for radial growthabout the fiber. For fiber materials that have distinct sides such astapes, mats, fabrics, plies, and the like, catalyst can be disposed onone or both sides and correspondingly, CNTs can be grown on one or bothsides as well.

As described above, CNS-synthesis can be performed at a rate sufficientto provide a continuous process for functionalizing spoolable fibermaterials. Numerous apparatus configurations facilitate such continuoussynthesis and result in the complex CNS morphology, as exemplifiedbelow.

One configuration for continuous CNS synthesis involves an optimallyshaped (shaped to match the size and shape of the substrate) reactor forthe synthesis and growth of carbon nanotubes directly on fibermaterials. The reactor can be designed for use in a continuous in-lineprocess for producing CNS-bearing fibers. In some embodiments, CNSs canbe grown via a chemical vapor deposition (“CVD”) process at atmosphericpressure and at elevated temperature in the range of about 550° C. toabout 800° C. in a multi-zone reactor. The fact that the synthesisoccurs at atmospheric pressure is one factor that facilitates theincorporation of the reactor into a continuous processing line forCNS-on-fiber synthesis. Another advantage consistent with in-linecontinuous processing using such a zoned reactor is that CNT growthoccurs in a seconds, as opposed to minutes (or longer) as in otherprocedures and apparatus configurations typical in the art.

CNS synthesis reactors in accordance with the various embodimentsinclude the following features:

Optimally Shaped Synthesis Reactors: Adjusting the size of the growthchamber to more effectively match the size of the substrate travelingthrough it improves reaction rates as well as process efficiency byreducing the overall volume of the reaction vessel. The cross section ofthe optimally shaped growth chamber can be maintained below a volumeratio of chamber to substrate of 10,000. In some embodiments, the crosssection of the chamber is maintained at a volume ratio of below 1,000.In other embodiments, the cross section of the chamber is maintained ata volume ratio below 500.

Although gas deposition processes, such as CVD, are typically governedby pressure and temperature alone, volume has a significant impact onthe efficiency of deposition. By matching the shape of the substratewith the growth chamber there is greater opportunity for productive CNSforming reactions to occur. It should be appreciated that in someembodiments, the synthesis reactor has a cross section that is describedby polygonal forms according the shape of the substrate upon which theCNS is grown to provide a reduction in reactor volume. In someembodiments, gas can be introduced at the center of the reactor orwithin a target growth zone, symmetrically, either through the sides orthrough the top and bottom plates of the reactor. This improves theoverall CNT growth rate because the incoming feedstock gas iscontinuously replenishing at the hottest portion of the system, which iswhere CNT growth is most active. This constant gas replenishment is animportant aspect to the increased growth rate exhibited by the shapedCNT reactors.

Zoning: Chambers that provide a relatively cool purge zone depend fromboth ends of the synthesis reactor. Applicants have determined that ifhot gas were to mix with the external environment (i.e., outside of thereactor), there would be an increase in degradation of most fibermaterials. The cool purge zones provide a buffer between the internalsystem and external environments. Typical CNT synthesis reactorconfigurations known in the art typically require that the substrate iscarefully (and slowly) cooled. The cool purge zone at the exit of thepresent CNS growth reactor achieves the cooling in a short period oftime, as required for the continuous in-line processing.

Non-contact, hot-walled, metallic reactor: In some embodiments, ahot-walled reactor made of metal can be employed, in particularstainless steel. This may appear counterintuitive because metal, andstainless steel in particular, is more susceptible to carbon deposition(i.e., soot and by-product formation). Thus, most CNT reactorconfigurations use quartz reactors because there is less carbondeposited, quartz is easier to clean, and quartz facilitates sampleobservation.

However, it has been observed that the increased soot and carbondeposition on stainless steel results in more consistent, faster, moreefficient, and more stable CNT growth. Without being bound by theory ithas been indicated that, in conjunction with atmospheric operation, theCVD process occurring in the reactor is diffusion limited. That is, thecatalyst is “overfed;” too much carbon is available in the reactorsystem due to its relatively higher partial pressure (than if thereactor was operating under partial vacuum). As a consequence, in anopen system—especially a clean one—too much carbon can adhere tocatalyst particles, compromising their ability to synthesize CNTs. Insome embodiments, the rectangular reactor is intentionally run when thereactor is “dirty,” that is with soot deposited on the metallic reactorwalls. Once carbon deposits to a monolayer on the walls of the reactor,carbon will readily deposit over itself. Since some of the availablecarbon is “withdrawn” due to this mechanism, the remaining carbonfeedstock, in the form of radicals, react with the catalyst at a ratethat does not poison the catalyst. Existing systems run “cleanly” which,if they were open for continuous processing, would produce a much loweryield of CNTs at reduced growth rates.

Although it is generally beneficial to perform CNT synthesis “dirty” asdescribed above, certain portions of the apparatus, such as gasmanifolds and inlets, can nonetheless negatively impact the CNT growthprocess when soot created blockages. In order to combat this problem,such areas of the CNT growth reaction chamber can be protected with sootinhibiting coatings such as silica, alumina, or MgO. In practice, theseportions of the apparatus can be dip-coated in these soot inhibitingcoatings. Metals such as INVAR® can be used with these coatings as INVARhas a similar CTE (coefficient of thermal expansion) ensuring properadhesion of the coating at higher temperatures, preventing the soot fromsignificantly building up in critical zones.

In some embodiments, the reaction chamber may comprise SiC, alumina, orquartz as the primary chamber materials because they do not react withthe reactive gases of CNS synthesis. This feature allows for increasedefficiency and improves operability over long durations of operation.

Combined Catalyst Reduction and CNS Synthesis. In the CNT synthesisreactor, both catalyst reduction and CNS growth can occur within thereactor. This feature is significant because the reduction operationcannot be accomplished timely enough for use in a continuous process ifperformed as a discrete operation. In typical carbon nanotube synthesisprocesses, catalyst reduction typically takes 1-12 hours to perform. Insynthesizing a carbon nanostructure according to the embodimentsdescribed herein, both catalyst reduction and CNS synthesis occur in thereactor, at least in part, due to the fact that carbon feedstock gas isintroduced at the center of the reactor, not the end as would typicallybe performed using cylindrical reactors. The reduction process occurs asthe fibers enter the heated zone; by this point, the gas has had time toreact with the walls and cool off prior to reacting with the catalystand causing the oxidation-reduction (via hydrogen radical interactions).It is this transition region where the reduction occurs. At the hottestisothermal zone in the system, the CNS growth occurs, with the greatestgrowth rate occurring proximal to the gas inlets near the center of thereactor.

In some embodiments, when loosely affiliated fiber materials, such astow are employed, the continuous process can include operations thatspreads out the strands and/or filaments of the tow. Thus, as a tow isunspooled it can be spread using a vacuum-based fiber spreading system,for example. When employing sized fibers, which can be relatively stiff,additional heating can be employed in order to “soften” the tow tofacilitate fiber spreading. The spread fibers which comprise individualfilaments can be spread apart sufficiently to expose an entire surfacearea of the filaments, thus allowing the tow to more efficiently reactin subsequent process steps. Such spreading can approach between about 4inches to about 6 inches across for a 3 k tow. The spread tow can passthrough a surface treatment step that is composed of a plasma system asdescribed above. After a barrier coating is applied and roughened,spread fibers then can pass through a CNT-forming catalyst dip bath. Theresult is fibers of the tow that have catalyst particles distributedradially on their surface. The catalyzed-laden fibers of the tow thenenter an appropriate CNT growth chamber, such as the optimally shapedchamber described above, where a flow through atmospheric pressure CVDor PE-CVD process is used to synthesize the CNS at rates as high asseveral microns per second. The fibers of the tow, now with radiallyaligned CNTs in the form of the CNS morphology, exit the CNT growthreactor.

In some embodiments, CNS-laden fiber materials can pass through yetanother treatment process prior to isolation that, in some embodimentsis a plasma process used to functionalize the CNS. Additionalfunctionalization of CNS can be used to promote their adhesion toparticular resins. Thus, in some embodiments, the processes can provideCNS-laden fiber materials having functionalized CNS. Completing thisfunctionalization process while the CNS are still on the fiber canimprove treatment uniformity.

In some embodiments, a continuous process for growing of CNS onspoolable fiber materials can achieve a linespeed between about 0.5ft/min to about 36 ft/min. In this embodiment where the CNT growthchamber is 3 feet long and operating at a 750° C. growth temperature,the process can be run with a linespeed of about 6 ft/min to about 36ft/min to produce, for example, CNTs having a length between about 1micron to about 10 microns. The process can also be run with a linespeedof about 1 ft/min to about 6 ft/min to produce, for example, CNTs havinga length between about 10 microns to about 100 microns. The process canbe run with a linespeed of about 0.5 ft/min to about 1 ft/min toproduce, for example, CNTs having a length between about 100 microns toabout 200 microns. The CNT length is not tied only to linespeed andgrowth temperature, however, the flow rate of both the carbon feedstockand the inert carrier gases can also influence CNT length. For example,a flow rate consisting of less than 1% carbon feedstock in inert gas athigh linespeeds (6 ft/min to 36 ft/min) will result in CNTs having alength between 1 micron to about 5 microns. A flow rate consisting ofmore than 1% carbon feedstock in inert gas at high linespeeds (6 ft/minto 36 ft/min) will result in CNTs having length between 5 microns toabout 10 microns.

In some embodiments, more than one material can be run simultaneouslythrough the process. For example, multiple tapes tows, filaments, strandand the like can be run through the process in parallel. Thus, anynumber of pre-fabricated spools of fiber material can be run in parallelthrough the process and re-spooled at the end of the process. The numberof spooled fiber materials that can be run in parallel can include one,two, three, four, five, six, up to any number that can be accommodatedby the width of the CNT-growth reaction chamber. Moreover, when multiplefiber materials are run through the process, the number of collectionspools can be less than the number of spools at the start of theprocess. In such embodiments, strands, tows, or the like can be sentthrough a further process of combining such fiber materials into higherordered fiber materials such as woven fabrics or the like. Thecontinuous process can also incorporate a post processing chopper thatfacilitates the formation CNS-laden chopped fiber mats, for example.

The continuous processing can optionally include further CNS chemistry.Because the CNS is a polymeric network of CNTs, all the chemistriesassociated with individualized CNTs may be carried out on the CNSmaterials. Such chemistries can be performed inline with CNS preparationor separately. In some embodiments, the CNS can be modified while it isstill substrate-bound. This can aid in purification of the CNS material.In other embodiments, the CNS chemistry can be performed after it isremoved from the substrate upon which it was synthesized. Exemplarychemistries include those described herein above in addition tofluorination, oxidation, reduction, and the like. In some embodiments,the CNS material can be used to store hydrogen. In some embodiments, theCNS structure can be modified by attachment to another polymericstructure to form a diblock polymer. In some embodiments, the CNSstructure can be used as a platform for attachment of a biomolecule. Insome embodiments, the CNS structure can be configured to be used as asensor. In some embodiments, the CNS structure can be incorporated in amatrix material to form a composite material. In some embodiments, a CNSstructure can be modified with reagents known to unzip CNTs and formgraphene nanoribbons. Numerous other chemistries and downstreamapplications can be recognized by those skilled in the art.

In some embodiments, the processes allow for synthesizing a first amountof a first type of CNS on the fiber material, in which the first type ofCNS comprises CNTs selected to alter at least one first property of thefiber material. Subsequently, the processes allow for synthesizing asecond amount of a second type of CNS on the fiber material, in whichthe second type of CNS contains carbon nanotubes selected to alter atleast one second property of the fiber material.

In some embodiments, the first amount and second amount of CNTs aredifferent. This can be accompanied by a change in the CNT type or not.Thus, varying the density of CNS can be used to alter the properties ofthe original fiber material, even if the CNT type remains unchanged. CNTtype can include CNT length and the number of walls, for example. Insome embodiments the first amount and the second amount are the same. Ifdifferent properties are desirable along two different stretches of thefiber material, then the CNT type can be changed, such as the CNTlength. For example, longer CNTs can be useful in electrical/thermalapplications, while shorter CNTs can be useful in mechanicalstrengthening applications.

Electrical conductivity or specific conductance is a measure of amaterial's ability to conduct an electric current. CNTs with particularstructural parameters such as the degree of twist, which relates to CNTchirality, can be highly conducting, thus exhibiting metallicproperties. A recognized system of nomenclature for CNT chirality hasbeen formalized and is recognized by those skilled in the art. Thus, forexample, CNTs are distinguished from each other by a double index (n,m)where n and m are integers that describe the cut and wrapping ofhexagonal graphite so that it makes a tube when it is wrapped onto thesurface of a cylinder and the edges are sealed together. When the twoindices are the same, m=n, the resultant tube is said to be of the“arm-chair” (or n,n) type, since when the tube is cut perpendicular tothe CNT axis only the sides of the hexagons are exposed and theirpattern around the periphery of the tube edge resembles the arm and seatof an arm chair repeated n times. Arm-chair CNTs, in particular SWNTs,are metallic, and have extremely high electrical and thermalconductivity. In addition, such SWNTs have extremely high tensilestrength.

In addition to the degree of twist, CNT diameter also effects electricalconductivity. As described above, CNT diameter can be controlled by useof controlled size CNT-forming catalyst nanoparticles. CNTs can also beformed as semi-conducting materials. Conductivity in multi-walled CNTs(MWNTs) can be more complex. Interwall reactions within MWNTs canredistribute current over individual tubes non-uniformly. By contrast,there is no change in current across different parts of metallicsingle-walled nanotubes (SWNTs). Carbon nanotubes also have very highthermal conductivity, comparable to diamond crystal and in-planegraphite sheets. Any of these characteristic properties of CNTs can beexhibited in a CNS. In some embodiments, the CNS can facilitaterealization of property enhancements in materials in which the CNS isincorporated to a degree that is greater than that of individualizedCNTs.

Although the invention has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these are only illustrative of the invention. It should beunderstood that various modifications can be made without departing fromthe spirit of the invention. The invention can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the invention. Additionally,while various embodiments of the invention have been described, it is tobe understood that aspects of the invention may include only some of thedescribed embodiments. Accordingly, the invention is not to be seen aslimited by the foregoing description.

What is claimed is the following:
 1. A method for making a polymercomposite, the method comprising: combining a plurality of carbonnanostructures and a polymer matrix, the carbon nanostructures eachbeing free of an adhered growth substrate and comprising a plurality ofcarbon nanotubes that are branched, crosslinked, and share common wallswith one another; and dispersing the carbon nanostructures in thepolymer matrix under applied shear; wherein the applied shear breakscrosslinks between the carbon nanotubes to form a plurality of fracturedcarbon nanotubes that are dispersed as individuals in the polymermatrix; wherein the plurality of fractured carbon nanotubes comprisescarbon nanotubes that are branched and share common walls with oneanother.
 2. The method of claim 1, further comprising: hardening thepolymer matrix after dispersing the fractured carbon nanotubes asindividuals therein.
 3. The method of claim 1, wherein the carbonnanostructures are in the form of a carbon nanostructure flake materialbefore being dispersed under the applied shear.
 4. The method of claim1, wherein a concentration of fractured carbon nanotubes in the polymermatrix ranges between about 0.1% to about 15% by weight of the polymercomposite.
 5. The method of claim 1, wherein a concentration offractured carbon nanotubes in the polymer matrix ranges between about 3%to about 6% by weight of the polymer composite.
 6. The method of claim1, wherein at least a portion of the carbon nanostructures arefunctionalized before being dispersed under the applied shear.
 7. Themethod of claim 1, wherein at least a portion of the carbonnanostructures further comprise an additive associated therewith, theadditive becoming dispersed in the polymer matrix under the appliedshear.
 8. The method of claim 7, wherein the additive comprises acoating disposed on at least a portion of the carbon nanotubes of thecarbon nanostructure.
 9. The method of claim 8, wherein the coatingremains on the fractured carbon nanotubes once they are dispersed asindividuals in the polymer matrix.
 10. The method of claim 1, whereinthe carbon nanotubes in each carbon nanostructure are formed withbranching, crosslinking, and sharing common walls during formation ofthe carbon nanotubes on a growth substrate.
 11. The method of claim 1,further comprising: incorporating a fiber material in the polymermatrix.
 12. The method of claim 1, further comprising: forming thecarbon nanostructures on a growth substrate; and removing the carbonnanostructures from the growth substrate prior to combining the carbonnanostructures with the polymer matrix.
 13. The method of claim 12,wherein removing the carbon nanostructures from the growth substratetakes place by a technique selected from the group consisting of fluidshearing, mechanical shearing, chemical etching, sonication, and anycombination thereof.
 14. The method of claim 1, wherein the fracturedcarbon nanotubes are dispersed as individuals in the polymer matrixwithout a surfactant being present.